Frequency division multiplexing (FDM) is a technology for transmitting multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal is transmitted within its own unique frequency range on a carrier wave that is modulated by data (text, voice, video, etc.).
The orthogonal FDM (OFDM) modulation technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides “orthogonality”, which prevents the demodulators from detecting frequencies other than their own.
Orthogonal frequency division multiplexing (OFDM) has been implemented in the wireless local-area-network (WLAN) IEEE 802.11 as well as digital audio broadcasting (DAB) and asymmetric digital subscriber line (ADSL) standards.
The main advantages of OFDM are its high spectral efficiency and its ability to deal with frequency-selective fading and narrowband interference. The spectral efficiency of OFDM systems can be further increased by adding multiple antenna techniques, also known as multiple-input multiple-output (MIMO) techniques. The indoor deployment of WLANs makes MIMO OFDM a strong candidate for high throughput extensions of current WLAN standards because the throughput enhancement of MIMO is especially high in richly-scattered scenarios, of which indoor environments are typical examples.
The performance of OFDM systems is seriously affected by phase noise. Phase noise occurs due to the instability of the system local oscillator. This instability spreads the power spectral density (PSD) of the local oscillator across adjacent frequencies, rather than remaining concentrated at a particular frequency. This type of phase noise is common in conventional oscillators. Consequently, carriers generated by these oscillators are not strictly “orthogonal”, and therefore inter-carrier interference (ICI) and inter-symbol interference (ISI) occurs in the received signal.
Such phase noise resulting from a difference between the carrier frequency and the local oscillator is a limiting factor for OFDM system performance, particularly for high data rates. Phase noise can be seen as consisting of two components: a common phase error (CPE) component that is common to all carriers and a time varying component that is frequency dependent. The time varying component, which is typically weaker than the CPE component, generates ICI. CPE can be removed by averaging over all carriers. Methods for eliminating CPE are known to those skilled in the art, e.g., a technique proposed by Schenk et al. Further improvements in phase noise suppression are disclosed in U.S. patent application no. 2004/0190637 by Maltsev, et al. They combine pilot subcarriers of data symbols to generate an observation vector. After that, recursive filtering of the observation vectors is performed to generate the phase compensation estimate. Although the generation of observation vectors is based on received data, the found vector is constant during symbol time and therefore accounts only for common phase noise. Furthermore the compensation is done in the frequency domain and therefore cannot restore orthogonality when ICI is present.
There remains to be found a satisfactory method for suppressing ICI in OFDM systems. A common approach for ICI mitigation was presented by Wu and Bar-Ness. Another approach is time domain processing as described by Casas, et al. Casas proposes phase noise representation in a fixed basis independent of the system or specific external conditions. The dominant phase noise components are then estimated using least square (LS) fitting of several base vectors. When single base vector is used, the method reduces to that of Wu and Bar-Ness.
The method proposed by Casas is not an optimal solution because phase noise cannot be compactly represented in a fixed basis for all systems.
The present patent discloses a method where phase noise is represented in a system-dependent basis (derived from the received data or precalibrated) rather than in a fixed basis as in the prior art. This technique provides a significant reduction in phase noise and works well for strong phase noise. The system-dependent and time-dependent basis for phase noise representation that is proposed in the present patent is its primary innovation over previous methods. The present patent also discloses other possible enhancements of the technique for MIMO systems, such as online calibration and exploitation of null tones.